This theory of Ditto's existence in the Pokémon franchise explains that Ditto may possibly be a failed clone of Mew. The reasons to "confirm" this are listed below:
They are both pink. Although this does not prove much, it is possible that when Mew was being cloned in the creation of Mewtwo that the same color of Mew was necessary to make the Mewtwo-in-creation. Their shiny forms also both happen to be blue.
They both know transform, and can technically both learn any move. It is possible that Mewtwo was supposed to know transform to make a successful clone. However, the real Mewtwo does not know Transform.
They both weigh the same: 8.8 lbs. To make a successful clone, perhaps it was necessary for the two to weigh the same.
Ditto's appearance of a scientific blob - Ditto appears to have been man-made. However, there are more than one Ditto, speculating that it may reproduce asexually.
Mewtwo's Bio - Mewtwo was said to be the only successful copy of Mew, speculating that Ditto may be a failed clone.
Ditto in the Pokémon Mansion. In Pokémon Yellow, the wild Pokémon roster for the burned out Pokémon Mansion on Cinnabar Island (where Mewtwo was implied to have been created) was updated to feature Ditto, speculating that Ditto may have been created in the Mansion as well. This change was carried over to Fire Red and Leaf Green, the remakes of the original Pokémon Red and Blue.
Mew is said to contain the DNA of every single Pokemon, while Ditto can essentially breed with every single Pokémon (with very few exceptions outside of legendary Pokémon).
Both Mew and Ditto have base stats that are the same in every category. Mew has 100 base in each stat, while Ditto has 48 base. The failed cloning process may have resulted in such diminished stats.